Chitosan and gelatin based edible films: state diagrams, mechanical and permeation properties

Abstract

Films of chitosan and gelatin were prepared by casting their aqueous solutions (pH≈4.0) at 60°C and evaporating at 22 or 60°C (low- and high-temperature methods, respectively). The physical (thermal, mechanical and gas/water permeation) properties of these composite films, plasticized with water or polyols, were studied. An increase in the total plasticizer content resulted in a considerable decrease of elasticity modulus and tensile strength (up to 50% of the original values when 30% plasticizer was added), whereas the percentage elongation increased (up to 150% compared to the original values). The low-temperature preparation method led to the development of a higher percentage renaturation (crystallinity) of gelatin which resulted in a decrease, by one or two orders of magnitude, of CO₂ and O₂ permeability in the chitosan/gelatin blends. An increase in the total plasticizer content (water, polyols) of these blends was found to be proportional to an increase in their gas permeability.

Introduction

Chitin is the second most abundant naturally occurring biopolymer (after cellulose) and is found in the exoskeleton of crustaceans, in fungal cell walls and in other biological materials (Andrady and Xu, 1997). It is mainly poly(β-(1-4)-2-acetamido-d-glucose), which is structurally identical to cellulose except that a secondary hydroxyl on the second carbon atom of the hexose repeat unit is replaced by an acetamide group. Chitosan is derived from chitin by deacetylation in the presence of alkali. Therefore, chitosan is a copolymer consisting of β-(1-4)-2-acetamido-d-glucose and β-(1-4)-2-amino-d-glucose units with the latter usually exceeding 80%. Chitosans are described in terms of the degree of deacetylation and average molecular weight and their importance resides in their antimicrobial properties in conjunction with their cationicity and their film-forming properties (Muzzarelli, 1996). Film-making conditions, including solvent pH, ionic strength, type of solvent (acid) used and annealing treatment, are parameters often manipulated to alter the mechanical properties and membrane porosity. Ionic strength or pH can be manipulated in order to reduce inter- and intramolecular electrostatic repulsion between chitosan chains, thus allowing the chains to approach each other and enhance the inter- and intramolecular hydrogen bonding (Chen et al., 1994). Chitosan has been extensively used over a wide range of applications, such as a biomaterial in medicine either on its own or as a blend component (Zhang et al., 1997; Hasegawa et al., 1992a, Hasegawa et al., 1992b, Hasegawa et al., 1994), a membrane filter for water treatment (Kawamura, 1995; Muzzarelli, 1977), a biodegradable, edible coating or film in food packaging (Wong et al., 1992; El Ghaouth et al., 1991a, El Ghaouth et al., 1991b; Butler et al., 1996) a dietary fibre, and a medicine against hypertension because of its scavenging action for chloride ions (Okuda, 1995; Furda and Brine, 1990; Muzzarelli, 1996).

It is usually little appreciated that collagen constitutes as much as 30% of total human protein with similar proportions being found in most animals. It is widespread in both vertebrates and invertebrates from primitive marine worms to mammals, providing strength and support for the animals' tissues and organs. The collagen molecule exists as a triple helix, comprising three discrete α-chains (three-dimensional structure), which leaves space for interchain hydrogen bonding. The existing imino acids impart rigidity to the molecule and the interstitial water molecules might act as hydrogen-bond bridges, thus contributing to the stability of the helix. The three α-chains of collagen are not identical but have slight variations in their imino acid content (Johnston-Banks, 1990). To convert insoluble collagen into soluble gelatin, the primary structure of which closely resembles that of the parent collagen, acid or alkaline pretreatments are required for cleaving a sufficient number of covalent cross-links in the collagen. Partial removal of amide groups results in increase of carboxyl groups in the gelatin molecule, thus lowering the isoelectric point.

The three-dimensional gel network of gelatin is composed of microcrystallites interconnected with amorphous regions of randomly coiled segments (Slade and Levine, 1987). Gelatin's ability to form thermoreversible gels with a melting point close to body temperature (Achet and He, 1995) has contributed substantially to an increase in its applications. Gelatin's largest single food use is in gel desserts because of the unique `melt at mouth temperature' (Slade and Levine, 1987; Kalafatas et al., 1975; Johnston-Banks, 1990), in frozen foods and in dairy products as a protective colloid or stabilizer, i.e. ice crystal inhibitor (Fiscella, 1983; Morley, 1984). Gelatin has also been used in photographic emulsions, playing a multipurpose role such as a protective colloid, ripening agent and binder (Jolley, 1970), in the textile industry as an adhesive (Bradbury and Martin, 1952) and in the pharmaceutical industry for the production of tablets and hard capsules (Healey et al., 1974; Johnson, 1965). Food coating and casing applications such as sausage casings and poultry coatings, with or without the presence of antimicrobial compounds, are envisaged as another important and promising issue which has primarily received attention by the meat industry (Keil and Hills, 1961; Keil et al., 1960; Hood, 1987; Moorjani et al., 1978; Klose et al., 1952). Occasionally, gelatin has been used in conjunction with other hydrocolloids such as acacia (gum arabic), alginate and pectate esters, soluble and hydroxy propyl starch (Deasy, 1984; McKay et al., 1985; Arvanitoyannis et al., 1997a, Arvanitoyannis et al., 1997b). Blending of chitosan with other hydrophilic polymers such as poly(vinyl alcohol), poly(vinyl pyrrolidone) or pectin, occasionally followed by alkali cross-linking, has been suggested as a promising avenue for the production of `tailor-made' blends (Blair et al., 1987; Quarashi et al., 1992; Yao et al., 1996; Andrady, 1992; Suto and Ui, 1996).

The aim of this article is to investigate the properties of binary or ternary/quartenary single-phase blends consisting of a protein–polysaccharide matrix plasticized with water and/or other polyols and to correlate these results with those reported in the literature for materials of similar resources.